1. Field of the Invention
The present invention pertains to an improved process for producing a carbonaceous pitch product having a mesophase content ranging from about 50 to 100%, which is suitable for carbon fiber manufacture. More particularly, the invention relates to a process for making mesophase containing pitch capable of producing carbon fibers having enhanced properties, by contacting a feedstock with an oxidative gas at an elevated temperature to prepare a mesophase precursor substantially free from mesophase and thereafter subjecting the mesophase precursor to heat treatment in melt phase at a higher temperature in the presence of a non-reactive sparge gas.
2. The Prior Art
In recent years extensive patent literature has evolved concerning the conversion of carbonaceous pitch feed material into a mesophase-containing pitch which is suitable for the manufacture of carbon fibers having desirable modulus of elasticity, tensile strength, and elongation characteristics.
U.S. Pat. No. 4,209,500 (issued to Chwastiak) is directed to the production of a high mesophase pitch that can be employed in the manufacture of carbon fibers. This patent is one of a series of patents pertaining to a process for producing mesophase pitches suitable for carbon fiber production. Each of these patents broadly involves heat treating or heat soaking the carbonaceous feed while agitating and/or passing an inert gas therethrough so as to produce a more suitable pitch product for the manufacture of carbon fibers.
As set forth in the Chwastiak patent, earlier U.S. Pat. Nos. 3,976,729 and 4,017,327 issued to Lewis et al involve agitating the carbonaceous starting material during the heat treatment. The use of an inert sparge gas during heat treatment is found in U.S. Pat. Nos. 3,974,264 and 4,026,788 issued to McHenry. Stirring or agitating the starting material while sparging with an inert gas is also disclosed in the McHenry patents.
U.S. Pat. No. 3,595,946 (Joo et al) discloses heat treating and distilling coal tar pitch to increase its average molecular weight by polymerization. Various oxidizing, dehydrogenating and polymerization agents may be employed to expedite the process. The treated pitch is spun into filament which is oxidized and then carbonized.
U.S. Pat. No. 4,474,617 (Nemura et al) describes treating low mesophase content pitch with oxidizing gas at a temperature of 200.degree. to 350.degree. C. to produce an improved carbon fiber.
Japanese Patent 65090 (Yamada et al) describes making a mesophase pitch for carbon fiber manufacture by heat treating feed in the presence of oxidizing gas at 350.degree. to 500.degree. C.
In published German Patent Application No. 3305-055-A (Nippon Oil KK) there is disclosed a process wherein a pitch feed is initially heat treated at 370.degree. to 420.degree. C. in a stream of inert gas for 5 to 20 hours under atmospheric or reduced pressure. Subsequently, an oxidant gas such as air or oxygen is passed through the pitch at 200.degree.-350.degree. C., one atmosphere pressure, at a flow rate of 1.0 to 3.5 SCFH for 10 minutes to 2 hours.
Koppers Co. Inc. has published DT 2221707-Q and DT 2357477 patent applications, which disclose manufacture of isotropic carbon fibers wherein the starting material is first reacted with oxygen and then vacuum distilled, to remove non-oxidized lower-boiling components.